The invention relates to a diffractive optical element and also an optical arrangement comprising a diffractive optical element.
A diffractive optical element is disclosed in the specialist paper “Zonal diffraction efficiencies and imaging of micro-Fresnel lenses” in J. Mod. Opt., 45, 1405, 1998. The latter proposes reducing the structural heights of the diffraction structures with increasing radius of the Fresnel lenses therein, that is to say with decreasing structural width, and this, in the case of the optical boundary conditions chosen therein, results in an increase in the diffraction efficiency at the lens rim.
In the case of many diffractive optical elements in which, as is still being discussed, the diffraction efficiency decreases with smaller structural width, such a variation in the structural height does not, however, result in an improvement in the diffraction efficiency, with the result that the teaching of the specialist paper cannot be generalized. A diffractive optical element having constant structural heights is disclosed in U.S. Pat. No. 5,623,365 A. One of the transmissive diffractive optical elements described therein has the function of a lens having a certain focal length. This necessitates that the widths of the diffraction structures become smaller with increasing distance from the central point. The greater the desired refracting power of such a diffractive optical element with a given refractive index is to be, the greater becomes the variation in the widths of the diffractive structures with the distance from the central point and, consequently, the variation in the ratio of said widths and of the wavelength, which ratio is mainly responsible for the achievable local diffraction efficiency.
In the case of a diffractive optical element according to the type of U.S. Pat. No. 5,623,365 A, said variation in the ratio of structural width and wavelength manifests itself, as calculations based on the electromagnetic diffraction theory have shown, in that the more light is diffracted into other orders of diffraction, the narrower are the diffraction structures. This results in losses in the local diffraction efficiency in the region of narrower diffraction structures, and this results in a variation, usually undesirable, in the local diffraction efficiency of the diffractive optical element.
EP 0 312 341 A2 describes a transmissive diffractive optical element that has a plurality of concentrically disposed diffraction regions that are each designed for different illumination wavelengths and within which there is a constant structural height. Within each of said diffraction regions, therefore, the disadvantages explained in connection with U.S. Pat. No. 5,623,365 A also occur here in the case of a variation of the structural widths, and this affects the dependence of the local diffraction efficiency.
The diffractive optical element in EP 0 312 341 A2 may, in addition, have annular zones that are of opaque or partially transparent design in order to modify the light passing through so as to achieve a desired intensity distribution in the beam path downstream of the diffractive optical element. EP 0 312 341 A2 consequently discloses an optical arrangement wherein the constant structural heights of the diffraction structures for an illumination wavelength also result in the disadvantages that were discussed in connection with U.S. Pat. No. 5,623,365 A.
It is therefore a first object of the present invention to develop a diffractive optical element in such a way that its local diffraction efficiency is optimally adapted to the application purpose.
Said object is achieved, according to the invention, by a diffractive optical element having the features of the present invention.